1. Field of the Invention
This invention relates to the fabrication of magnetic read/write heads that employ TAMR (thermally assisted magnetic recording) to enable writing on magnetic media having high coercivity and high magnetic anisotropy. More particularly, it relates to the use of a plasmon antenna (PA) to transfer optically induced plasmon energy from the read/write head to the media.
2. Description of the Related Art
Magnetic recording at area data densities of between 1 and 10 Tera-bits per in2 (Tbpsi) involves the development of new magnetic recording mediums, new magnetic recording heads and, most importantly, a new magnetic recording scheme that can delay the onset of the so-called “superparamagnetic” effect. This effect is the thermal instability of the extremely small magnetic regions on which information must be recorded in order to achieve the required data densities. A way of circumventing this thermal instability is to use magnetic recording mediums with high magnetic anisotropy and high coercivity that stabilize the recorded regions against thermal perturbations, yet can still be written upon by the magnetic fields of the increasingly small write heads required for producing the high data density. This way of addressing the problem produces two conflicting requirements:    1. the need for a stronger writing field that is necessitated by the highly anisotropic and coercive magnetic mediums and;    2. the need for a smaller write head of sufficient definition to produce the high areal write densities, which write heads, disadvantageously, produce a smaller field gradient and broader field profile.
Satisfying these requirements simultaneously may be a limiting factor in the further development of the present magnetic recording scheme used in state of the art hard-disk-drives (HDD). If that is the case, further increases in recording area density may not be achievable within those schemes. It should be noted that these conflicts are not restricted to the typical magnetic field writer configurations in which the write head is mounted within a slider that flies above the magnetic medium at distances of 0.5 nm or greater. Indeed, other methods for writing on a magnetic medium, such as the use of near-contact writers in which the head-to-disk clearance may be 0.5 nm or less, also fall victim to the fact that the small writers required for the high resolution writing do not produce sufficient field strengths to overcome the coercivity and anisotropy of the recording medium.
One way of addressing these conflicting requirements, wherever they arise, is by the use of assisted recording methodologies, notably thermally assisted magnetic recording, or TAMR.
The prior art forms of assisted-recording methodologies being applied to the elimination of the above problem share a common feature: transferring energy into the magnetic recording system through the use of physical methods that are not directly related to the magnetic field produced by the write head. If an assisted recording scheme can produce a medium-property profile to enable low-field writing localized at the write field area, then even a weak write field can produce high data density recording because of the multiplicative effect of the spatial gradients of both the medium property profile and the write field. These prior art assisted-recording methods either involve deep sub-micron localized heating by an optical beam or ultra-high frequency AC magnetic field generation.
The heating effect of TAMR works by raising the temperature of a small region of the magnetic medium to essentially its Curie temperature (TC), at which temperature both its coercivity and anisotropy are significantly reduced and magnetic writing becomes easier to produce within that region.
Recently, a particular implementation of TAMR has appeared, namely the transfer of energy from electromagnetic radiation to a small, sub-micron sized region of a magnetic medium through interaction with the near field of an edge or surface plasmon. In this case the electromagnetic radiation is radiation in the range of optical frequencies and that optical frequency radiation is excited by an optical frequency laser. The plasmon is excited in a small conducting plasmon generator (PG) that is incorporated within the read/write head structure. The particular source of optical excitation can be a laser diode, also contained within the read/write head structure, or a laser source that is external to the read/write head structure, either of which directs its beam at the antenna through a means such as an optical waveguide (WG). As a result of the WG, the optical mode of the incident radiation couples to a plasmon mode in the PG, whereby the optical energy is converted into plasmon energy, This plasmon energy is then focused by the PG onto the medium at which point the heating occurs. When the heated spot on the medium is correctly aligned with the magnetic field produced by the write head pole, TAMR is achieved.
As we shall see, to both obtain and maintain efficient coupling between the optical radiation carried by the WG and the plasmon mode excited within the PG, it is necessary to: (1) engineer the shape of the PG so that it matches well with the size of the WG for the coupling itself to be efficient, and (2) to condense the plasmon energy into a highly confined plasmon mode by incorporating and utilizing a self-focusing portion within the PG.
K. Tanaka et al. (US Publ. Pat. App. US2008/0192376) and K. Shimazawa et al. (US Publ. Pat. App. US2008/0198496) both describe TAMR structures that utilize edge plasmon mode coupling.
Ichihara et al (U.S. Pat. No. 6,741,524) discloses an electron emitter having a sharp tapered shape.
Referring first to FIG. 1a, there is shown a schematic illustration of an exemplary prior art TAMR structure in an air bearing surface (ABS) view and, in FIG. 1b, in a side cross-sectional view. The dimensional directions in the ABS view are indicated as x-y coordinates, with the x coordinate being a cross-track coordinate in the plane of the medium (ABS plane), the y coordinate being a down-track direction also in the plane of the medium and the z-direction being along the length of the magnetic pole, perpendicular to the ABS plane. As shown in both FIG. 1a and FIG. 1b, the conventional magnetic induction-type write head includes a main magnetic pole (MP) (1), which is shown with a rectangular ABS shape, a writer coil (5) (three winding cross-sections drawn) for inducing a magnetic field within the pole structure and a return pole (3) to complete the flux circuit. Generally, magnetic flux emerges from the main magnetic pole, passes through the magnetic media and returns through the return pole. As already mentioned above, nothing in this configuration should be considered as restricting the writer to any particular distance above the recording surface. The writer may be mounted in a slider that is configured to aerodynamically fly above the moving medium, or the slider may be a contact type slider that may be configured to virtually make contact with the recording medium. As long as the problem being addressed is one of enabling a small write field to overcome the coercivity and anisotropy of a magnetic medium, the following methodology will be applicable.
The waveguide (WG) (4) is an optical waveguide that guides optical frequency electromagnetic radiation (6) towards the ABS (10) of the write head. The ABS end of the write head will be denoted its distal end. The plasmon generator, PG (2) (note, we will refer to a device that propagates a plasmon as a “plasmon generator, PG”, whereas a device that supports a local plasmon mode may be referred to as a “plasmon antenna PA”), which has a triangular shape in the ABS plane, extends distally to the ABS. The distal end of the waveguide (4) terminates at a distance, d, which ranges from 0 to a few microns, away from the ABS. An optical frequency mode (6) of the electromagnetic radiation couples to the edge plasmon mode (7) of the PG (2) and energy from the edge plasmon mode is then transmitted to the media surface where it heats the surface locally at the ABS edge of the PG triangle. The local confinement of the edge plasmon mode is determined by the angle and dimensions of the triangular cross-section of the PG. Thus the PG can be engineered to achieve a very small light spot as well as a high temperature gradient in the medium, both of which are desirable in the TAMR recording scheme. Due to the long body and large metal volume of the PG, heating damage introduced to the PG during the writing process can be greatly reduced compared to an isolated PG of smaller volume. Referring finally to perspective schematic FIG. 1c, there is shown the PG (2), the WG (4) slightly above the vertex edge of the PG, the optical frequency radiation (downward directed arrows (6)) impinging upon the PG and the edge plasmon mode (7), shown as a horizontal arrow, propagating along the vertex of the PG.
It is always desirable to consume less optical laser power and, therefore, to pursue high optical efficiency so as to use as little power as possible in HDD operation utilizing TAMR. The prior arts have some critical limitations imposed on their use of optical power. First, the efficiency of coupling the waveguide radiation (6) to the plasmon mode (7) is limited due to the mismatch between the diffraction-limited optical mode (6) in the waveguide (WG) and the sub-diffraction limited edge plasmon mode (7) within the plasmon generator portion of the PG (2). Because the WG mode is much larger than the highly confined plasmon mode (7), only a small portion of the optical power can be transferred. Second, the propagation loss of the edge plasmon mode along the plasmon generator (2) could be significant due to the high confinement of the plasmon mode. Third, the coupling and propagation efficiencies of the edge plasmon mode tends to be highly sensitive to the variations of the edge shape of the PG, so tight fabrication tolerances are necessary.